PURPOSE: To call a terminating call side with simple circuit constitution by connecting the conference telephone set of a caller side to that of the terminating call side with the speech path of a system in current use and that of a spare system, and switching the speech path to that of the spare system when a fault occurs in the speech path of the system in current use.

CONSTITUTION: The conference telephone set 1 of the caller side transmits a specific tone signal representing a speech request outputted from a specific tone oscillation circuit 7 to the terminating call side via the speech path 4 of the spare system by switching a key 8, and the order telephone set 3 of the terminating call side detects the specific tone signal, and connects by switching the speech path to the speech path 4 of the spare system, while, outputs a specific tone by sending the specific tone signal to a speaker 18, and transmits the speech request to the terminating call side.